serious refactor: TileLoader:

- no more duplication of TileLoaders for GWT ->
- decouple loadTile() from TileDataSource completed() call
- all TileDataSource MUST call completed(success) in any case now
fix: ElementLayers, make sure setFrom() clears previous items
refactor: extract theme RulerBuilder from Rule
rename RenderThemeHandler - XmlThemeBuilder
cleanup: RenderTheme
rename from -> read
cleanup theme attribute validation
add TextBuilder for Text styles
cleanup: XmlThemeBuilder
start ThemeBuilder
theming changes:

- select="first" also add its renderstyles when no sub-rule matched
  ... before they where only added when one subrule matched

- use select="when-matched" as last subrule in select="first" rules
  instead.
docs
move TagFilter stuff to OsmTileLayer
refactor: TileLoader.createLoader()
get theme from VectorTileLayer, instead of setting it on Loader
refactor: MapTile hold generic TileData - Testing
start TileLoaderHook:

- only create ExtrusionLayers when a BuildingLayer is attached
fixup loader create
add TileData.owner as identifier
fix: inner class should be static