Add various keybindings

Taken from Geany:

• Convert > To Opposite Case: Ctrl + Alt + U
• Move Selection > Line Up: Alt + Page Up
• Move Selection > Line Down: Alt + Page Down
• Increase Indent: Ctrl + I
• Decrease Indent: Ctrl + U

Also, I noticed that Select All does not have a visible menu accel (Ctrl + A). Same goes for Delete (Del), but that one may be optional.