See https://github.com/qgis/QGIS-Enhancement-Proposals/issues/299
Implements the API framework for setting advanced labeling engine
rules for a project, and implements 4 initial rule types:
- QgsLabelingEngineRuleMinimumDistanceLabelToFeature: prevents labels
being placed too *close* to features from a different layer
- QgsLabelingEngineRuleMaximumDistanceLabelToFeature: prevents labels
being placed too *far* from features from a different layer
- QgsLabelingEngineRuleMinimumDistanceLabelToLabel: prevents labels
being placed too close to labels from a different layer
- QgsLabelingEngineRuleAvoidLabelOverlapWithFeature: prevents labels
being placed overlapping features from a different layer
(note that the first 3 rules require a build based on GEOS >= 3.10,
they are not available for older GEOS builds)
Also implements a registry for storing available rule classes,
and serialization of rules and configuration in QGIS projects
Shows the text bounds of the label in red and baselines in blue.
Designed for debugging purposes only.
Also move label engine enums to Qgis, promote to enum class
settings and logic
The previous approach of a single fixed value which applied to ALL line and ALL polygon
features was... not ideal. It meant that all line features would be assigned the same
number of candidates, regardless of length. So a road of length 1 cm on the rendered
map would have an identical number of candidates as a 30cm road covering the length of the
whole map!! This resulted in both a lot of wasted calculations (generating a ridiculous
number of candidates for small lines at barely discernable distances from each other)
AND an insufficient number of candidates for lengthy features (resulting in worse label
placement for these features).
(The situation was similar, but even worse for polygons)
Now, the setting is reworked to "Number of line candidates per cm" and "number of
polygon candidates per cm2". This means that small features get much less candidates,
and large features get much more features! Both a win for map rendering speed in many
circumstances AND good cartography... now that's a nice Christmas gift for QGIS :)
So that these files are all grouped together, making it easier to locate
all the components of the labeling engine and hopefully making things
easier to navigate
