This commit groups a set of new tools that were tested in grease pencil object branch before moving to master. We decide to do all the development in a separated branch because it could break master during days or weeks before the new tools were ready to deploy. The commit includes: - New Cutter tool to trim strokes and help cleaning up drawings. - New set of constraints and guides to draw different types of shapes. All the credits for this development goes to Charlie Jolly (@charlie), thanks for your help! - Segment selection mode to select strokes between intersections. - New operator to change strokes cap mode. - New option to display only keyframed frames. This option is very important when fill strokes with color. - Multiple small fixes and tweaks. Thanks to @pepeland and @mendio for their ideas, tests, reviews and support. Note: Still pending the final icons for Cutter in Toolbar and Segment Selection in Topbar. @billreynish could help us here?