Support drag and drop of PNG file

[markummitchell]

PNG file dragged into Engauge should be imported.

[markummitchell]

@k1moradi What release and platform are you using? I have tested on various systems and did not see the problem. If in linux, can you try installing a package named living or similar?

[Correction] Windows had drag and drop disabled. I have enabled it in a pre-release for testing at https://ci.appveyor.com/api/buildjobs/dlekc811twwebpiq/artifacts/engauge-build-x64.7z

[k1moradi]

I reproduced this bug on windows 10 64 bits. I tested 11.2 without installer and also 11.3 version that I compiled myself.

Just drag a PNG file on top of the program main window and drop it.

Nothing happens while it could invoke the import function.

[markummitchell]

Can you try the pre-release? Sorry about that, I tested the pre-release in Windows versus the version you had.

[k1moradi]

It works now! Thanks a lot.

PS: Theming is different when I compile from source on windows.
For example, the cascade menu on left (my build) is white, but the right one (your prerelease) is grey.

[markummitchell]

Great

The theme issue could be from the different themes (see #336), or just different build systems.