Add object mirror (horizontal/vertical) feature

Will also mirror directions when things are mirrored, and flip lines when not in vertices mode. Default shortcut keys Ctrl+M for horizontal and Ctrl+Shift+M for vertical. Also added to toolbar and menus.
Add mirror checkboxes to object edit panel
Don't draw line lengths on object edit

Doesn't really work with mirroring, and gets kinda cluttered anyway - might make it so that when you mouse over a line it shows the length instead
Shift+click during paste doesn't end the paste operation

Allows something to be copy+pasted multiple times more easily
Add option to remove invalid lines on sector delete
Update lines VBO when any line is modified
Merge branch 'master' of https://github.com/sirjuddington/SLADE into Music