Better input sizing on ask dialog

Looked into #786, and found that the ask dialog was hand-rolled with a bunch of SWT code. Ouch :hurtrealbad:!

Turns out, though, that with a [relatively simple addition of modal window support to the apps](bf05214), we can implement the ask in Shoes directly itself! :tada: 

**Old Ask**
![image](https://cloud.githubusercontent.com/assets/130504/4963402/65302d00-66cd-11e4-8bcd-11ee3735d0a2.png)

**New Ask**
![image](https://cloud.githubusercontent.com/assets/130504/4963397/33fbf23c-66cd-11e4-9688-8c443cdfbd16.png)

Very up for feedback on the exact sizing, appearance, and behavior of the ask dialog itself. Presently it looks *mostly* like the existing one to me on Mac, but I don't know whether OS defaults will leave it looking good elsewhere.

It also currently hardcodes it's height, so with enough text you will get scrolling on the full window itself, which is better than things were, but not totally awesome. Eventually once we get positioning immediately on adding elements we can probably do something better... not sure there's a better solution otherwise at the moment.