"Add Part" Tool Button

This pull request includes a few layout and cosmetic changes to the UI.

1. Collapsed the "Add {Honeycomb, Square, etc}" toolbar buttons into a [QToolButton](qt-project.org/doc/qt-5/qtoolbutton.html). The reason for this change is that it will give us much more flexibility for cleanly adding additional part types moving forward. (**NOTE:** Unfortunately QToolButton is not supported in Qt Creator, so future UI changes must be done manually in the ui_mainwindow.py source.)
2. Updated the makefile for running pyuic5 and pyrcc5 to rebuild the pyqt ui files (mainwindow and icons).
3. Updated the "Mod" icon for more consistent style.