Pre-adapted some of the widgets
Enabled mouse button handling
Changed Configurable class

It makes more sense to assign real attributes once
instead of searching for proper value every time attribute is requested
If given attribute has a setter then it will be used
Removed redundant assignments
Naming changes, refactored padding
Broke excessive lines
Adapted Image widget
Merged redundant Textbox class
Added constant module
Corrected children size calculation logic