Keyboard shortcuts

Note to developers: this PR introduced support for configurable keyboard shortcuts in Thorium.

• CTRL + F10 (FocusMain): in reader and library windows, moves the keyboard focus to the "main" area landmark (in the reader view, this is immediately before the frame that displays publication content)
• CTRL + t (FocusToolbar): in reader and library windows, moves the keyboard focus to the top horizontal toolbar, immediately before the "skip" link (which itself moves focus to the "main" area landmark)
• CTRL + n (FocusReaderNavigation): in reader window, moves the keyboard focus to the navigation panel (table of contents, bookmarks, etc.)
• CTRL + s (FocusReaderSettings): in reader window, moves the keyboard focus to the settings panel (user preferences for displaying publication content)
• CTRL + b (ToggleBookmark): in reader window, toggle bookmark (a notification is produced so that screen readers announce the change)
• CTRL + F11 (ToggleReaderFullscreen): in reader window, toggle fullscreen
• CTRL + i (OpenReaderInfo): in reader window, publication info popup modal dialog (may be used in future Thorium version for similar feature in library window)
• CTRL + w (CloseReader): in reader window, closes the window and returns to the library view.
• CTRL + f (FocusSearch): in library window, moves the keyboard focus to the search input text of local bookshelf or OPDS feed (may be used in future Thorium version for similar feature in reader window)
• CTRL + left or right (NavigatePreviousOPDSPage / NavigateNextOPDSPage): in library window for OPDS feeds, activates the previous or next pagination links
• CTRL + comma or period (NavigatePreviousOPDSPageAlt / NavigateNextOPDSPageAlt): same as above (alternative keyboard shortcut)
• CTRL + SHIFT + ALT (except on MacOS) + left or right (NavigatePreviousChapter / NavigateNextChapter): in reader window, move to previous or next "chapter" (spine item in the reading order of the publication). Note that the keyboard focus is automatically moved to the "main" area landmark, which is immediately before the frame that displays publication content (just like when following / activating a link in the table of contents).
• CTRL + page down or page up (NavigatePreviousChapterAlt / NavigateNextChapterAlt): same as above (alternative keyboard shortcut)
• left or right (NavigatePreviousPage / NavigateNextPage): in reader window, move to previous or next "page" (or scroll unit, if pagination is disabled)
• CTRL + comma or period (NavigatePreviousPageAlt / NavigateNextPageAlt): same as above (alternative keyboard shortcut)
• CTRL + 1 (AudioPrevious): in reader window, calls the "previous" command for audio (TTS or EPUB3 Media Overlays readaloud, or "rewind" for audiobooks)
• CTRL + 2 (AudioPlayPause): in reader window, calls the "play" or "pause" toggle commands for audio (TTS or EPUB3 Media Overlays readaloud)
• CTRL + 3 (AudioNext): in reader window, calls the "next" command for audio (TTS or EPUB3 Media Overlays readaloud, or "forward" for audiobooks)
• CTRL + 4 (AudioStop): in reader window, calls the "stop" command for audio (TTS or EPUB3 Media Overlays readaloud, or "pause" for audiobooks)