New algorithms and bugfixes

1. Bugs

• Plugin selection dialog showed "Exporter to used" -> changed to "Plugin to use"
• Fixed style of dropdowns
• Previously the border of a RegionLabel could only be changed when first hovering over the label; now it works also if approaching the label "from outside"
• Plotted channels are now the same before and after executing an algorithm

2. Additions

• Added example algorithm for analysis of existing labels
• Added example exporter
• Added BaseImporter.file_type to be able to give importer a preferred file name, such that only files of that type are shown when selecting data
• Added typecheck for passing plugins and labels to MainWindow

4. Readthedocs

• improved, e.g. includes working examples