Dckit

- fix: regression (in a793ed) where "medium" was not saved in the
user-defined meta data
- fix: in some cases there were two combo boxes for "medium"
in the integrity check dialog (12)
- fix: allow to remove items in integrity check dialog
- ref: refactored combobox selection in integrity check dialog
- setup: bump dclab from 0.32.2 to 0.32.5
- build: overhauled build pipeline

- setup: bump dclab from 0.29.1 to 0.32.2 (OOM issues for large
data files)

- fix: in some cases, .tdms datasets could not be converted to
.rtdc because of the data value of a combobox was not checked
(ValueError: could not convert string to float: 'please select')
- fix: joining datasets might have ended in an error because a
message box was not setup properly

- ci: migrate to GitHub Actions

- setup: correct installation directory on Windows
- setup: bump dclab from 0.28.0 to 0.29.1 (new image_bg feature,
relax fixed choices for [setup] 'medium')
- fix: main window not focused after startup
- enh: allow to edit [setup] 'medium' if it is set to "other"

- feat: add option to split large datasets
- setup: bump dcalb from 0.27.6 to 0.28.0 (splitting datasets)
- fix: file dialogs did not always immediately close when
OK was pressed