Unsaved indicator for DocumentControl

[ievgen-baida]

This PR adds possibility to mark individual document page as one having unsaved changes (HasUnsavedChanges). Such page would then be rendered with a little round indicator after the name of the document. The color can be configured using UnsavedBackgroundColor property.

Also, the calculations and drawing for each tab in ThemedDocumentControlHandler have been refactored:

• Introduce ImageRect and UsavedRect;
• Calculate rectangles once in CalculateTab (except for closeForegroundRect which is calculated while drawing it);
• Restructure calculations element by element;
• Each of the element has valid location and size (even though it can be empty);
• (Hopefully) simplify calculations by using PointF and RectangleF methods;
• Simplify drawing of the close button.

[cwensley]

Looks great, @ievgen-baida! Thanks for the contribution.