more keybindings and functionality

[claudeha]

Hi, thanks for Troop. I implemented some keys whose absence was frustrating me:

• Ctrl-Up previous blank line
• Ctrl-Down next blank line
• PageUp previous screenful
• PageDown next screenful
• Ctrl-MouseWheel adjusts font size

Only tested on Linux so far.

May conflict with https://github.com/Qirky/Troop/pull/27.

[Qirky]

Cool, will try and have a play around with this soon

[Qirky]

Nice work - merged :+1: