Generic Button Widget

Unifying button widgets throughout Elisa and making a unique widget properly documented that would support all our current use cases and those of the relook.

Current State In Elisa:

9 different button classes are existing (button in their name and inheriting from Widget directly):

[LOCALLY USED] pigment/widgets/osk.py:class Button(Widget):
[UNUSED] pigment/widgets/button.py:class Button(Widget):
[LOCALLY USED] poblesec/login_settings_controller.py:class ReturnButton(Widget):
[LOCALLY USED] poblesec/poblesec_browser_controller.py:class TopBarButton(Widget):
[LOCALLY USED] poblesec/viewport_buttons.py:class AutohideButtons(Widget):
[USED 'EXTERNALLY'] poblesec/widgets/player/button.py:class Button(Widget):
• Player Volume Bar poblesec/widgets/player/volumebar.py
• Player Back Button poblesec/player_video.py
• Player Control Ribbon
[USED EXTERNALLY] poblesec/widgets/button.py:class TextButton(Widget):
• Plugin Download&Install/Use poblesec/plugins.py
• Configuration Wizard poblesec/configuration/list.py
• Popups poblesec/popup.py
[USED EXTERNALLY] poblesec/widgets/button.py:class StateButton(Widget):
• Search Enter Button poblesec/search_controller.py
• Viewport Close/Minimise poblesec/viewport_buttons.py
[USED EXTERNALLY] poblesec/widgets/button.py:class IconButton(Widget):
• Menu item widget with action poblesec/widgets/menu_item.py