Add signature help

[famoser]

Adds signature help, but not quite ready:

• Disable signature help on definition, as leads to crash if new parameter is added
• Investigate whether can handle macro define/extend better
• Check definition really shows up in file with complex constructs (e.g. wrapped calls)
• Possibly find better solution to handle backspace to re-enable caching

[famoser]

Unresolved points:

• Unclear what target behavior of macros should be at the moment => Leave as is, taking over go to definition behavior
• Discovery is fast, even in bigger files. No easy way to support backspace => Leave caching disabled as it is now