Make dialog preview stylesheet default to dark background
Delete a useless function
Fix button label in one of the PC editor dialogs
Fix open game dialog not allowing valid save files to be selected
Legacy game saves now load, probably correctly
- Also fixed the file pick dialogs, which now return a path instead of a file:// URL
Little tweaks and stuff
- New file extension .boes for scenarios - intent is to use this for the new scenario format
- Turn on auto-enable for Application menu in the game (otherwise the Show/Hide options don't grey out correctly)
- Fix PC graphics not appearing in PC editor
Fix PC editor thinking a file is loaded even when the load failed
Fix a lot of bugs in the PC editor
- Alchemy dialog had black text on dark background
- Buttons didn't depress when clicked
- Edit XP button just said "Edit"
- Two menuitems in Special Edit menu had been left with default names
- Callbacks in Special Edit and Free Extras menu weren't correctly assigned
- Edit XP dialog put the XP to next level in the wrong place
- Fixed alchemy dialog LED naming

Supporting changes in the dialog engine:
- Getting or setting control text as a number now uses the long long type to allow maximal precision
- When adding a label to a button, the dialog's default text colour is used instead of the button's text colour if the dialog's background is dark
Add a few resources for future use:
- Graphics for scrollbars
- DejaVu Sans Book, for "plain" text (to replace Geneva Regular)
- Capriola Regular, for "bold" text (to replace Silom which replaced Geneva Bold)
Populate item menus in PC editor (they're not functional yet though)