More tweaks to the 2d polygon editor

Actually 3 changes that I think might be quite useful:

(1) A new option "Delete Points With Lmb" that allows point to be deleted via a simple short click on them. This is off by default, as mouse users will probably usually use the right mouse button to delete points. However, this is very useful for touch screens: you can now just tap on a point and thereby delete it using this option; only a very short tap without move produces a delete.

(2) A new lock option at the right hand of the editor toolbar, only visible while in edit point mode.:

Is enabled, locks the edges (or the number of points) for mouse editing; this means that if locked, you can no longer add or delete points with the "click on edge" or the new "click on point" functions. There might situations/phases/users where you really want to stick with the polygon point number as it is and only edit the positions (and might actually get annoyed if you add or delete points). This is what this lock is useful for, I believe: select it, and you only edit the positions. The lock setting is persistent across polygons meaning, you can adjust it to your workflow and edit a whole bunch of polys with the same lock setting.

(3) Fixes a small line overdraw glitch when drawing, i.e. this is fixed: