Fix bug #177, #178, and #180

[seagetch]

Fix for #177, #178
Drawable individually determine the actions based on cursor position and vertex positions in "update" method.

In some cases, actions for every drawables become differently in above algorithm.
e.g. One drawable moves vertices, while others are dragging areas to select vertices.

I broke down "update" method into "peek", "unify" and "update."
These methods are called in following order:

1. Actions are determined for every selected drawables by "peek" method based on vertex position and cursor position.
2. Determined actions are collected and passed to "unify"
3. "unify" method merges passed action into single action using predefined "priority of the action".
4. Determined action is passed to "update" method of every selected drawables again
5. actual action for every drawable is executed by "update"

In this manner, we can unify the action of all of the drawables.

Fix for #180
IncMeshEditor.setMirrorVert is not handled correctly.
I fixed the problem.

[seagetch]

Now we can move vertices of several drawable at once.

Midori-2023-02-13_21.19.46.mp4