Making calendar decorations reusable between calendar implementations (was: Re: September/October KDE PIM summary blog post)

[Fusion Future]

Hi,

I have implemented the alternate calendar feature by extending the 
calendar events plugin system. Please see the feature MR in 
kdeplasma-addons [1] and two other related framework MRs [2,3].

Also I have a question about whether it's possible to depend on external 
libraries in Plasma. Because currently QCalendar only supports a limited 
set of calendar systems [4], so it's inevitable to depend on the ICU 
library (or maintain a library in the plugin) to support more calendar 
systems, like Chinese Lunar Calendar system and Indian National Calendar 
system.

I also see there is a task to port away from Plasma Calendar [5]. In Qt 
6.3 QML has added Calendar type [6] and CalendarModel [7], but the 
feature is still limited.

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In general, the sub-label feature can also be useful in many other 
aspects, like showing holidays or emoji marks under the day numbers.

I would recommend adding the sub-label feature in the calendar events 
plugin [8], so there could be more possibilities to make the calendar 
applet more useful. The changes are relatively small and please provide 
some opinions on them. Thanks in advance!

[1] https://invent.kde.org/plasma/kdeplasma-addons/-/merge_requests/112
[2] https://invent.kde.org/frameworks/kdeclarative/-/merge_requests/112
[3] https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/450
[4] https://doc.qt.io/qt-5/qcalendar.html#System-enum
[5] https://phabricator.kde.org/T12112
[6] https://doc-snapshots.qt.io/qt6-6.3/qml-qtquick-controls2-calendar.html
[7] 
https://doc-snapshots.qt.io/qt6-6.3/qml-qtquick-controls2-calendarmodel.html
[8] https://invent.kde.org/frameworks/kdeclarative/-/issues/1