can select any subexpression with only left and right keys
window resize updates drawables
pairs with selected drawable or subexpression
attempt at new layout
variable panel added, also better layout
modified theorem panel layout (possibly unstable -- but committed due to probable imminent hardware failure)
improved layout, added context menus
Merge branch 'large-table-layout' into types

The merge is complete in a syntactic sense (Prove-It compiles and runs), but incomplete in a semantic sense.

Conflicts:
	src/Expression.java
	src/MainWindow.java