Clean up RCSS files

* Use automatic positioning via RCSS when possible
(middle of screen).
* messagebox.rml: Increase font size for content to 16px.

* UIContext: Introduce load_document_file.

* UIWidget: Rename ::desktop, ::set_desktop to ::context, ::set_context
and change argument type to UIContext.

* UIWidget: Rename ::alignment, ::set_alignment to ::text_alignment,
::set_text_alignment.

* UIWidget: Introduce ::set_alignment(uint32) for setting alignment of the
widget's document.

* UIWidget: Use the document's actual position, size values instead of
grabbing from RCSS properties; these are more accurate.
.gitignore: Use third-party/.gitignore for list of files to ignore
A bit of clean up in SDL2LoggerTest
Introduce NOM_ASSERT assertion helpers (SDL2 facility)
Clean up logging output in libRocket tests

Use NOM_LOG_CATEGORY_TEST for debug output logging && disable
logging of debug by default.
Fix practice of disabling NOM_ASSERT in test

* CardsPageDataSource: Fix several bugs in assert tests.
* CardsPageDataSource: Fix calculation for figuring out total number
of pages for even number of cards (i.e.: 110 cards).

* libRocketDataGridTest: Remove testing for invalid index positions.
Renderer: Change return type of ::logical_size to Size2i
Goodbye cruel world to pre-libRocket GUI codebase