feat: support for xiaomi smart pen

[arpandaze]

First of all I would like to thank you for this amazing project. Its perfect for my purpose. My only concern was that my Pen for Xiaomi Pad 6 wasn't supported. I took a look around the code base to implement support for the pen. Seems like you already have support for keyboard and since Xiaomi pen keys works as Page Up and Down, I have updated a few things to support the pen.

Changes made:

1. Implemented function to toggle between primary and secondary pinned colors.
2. Bind PageUp to toggle colors and PageDown to toggle eraser.
3. Workaround for keybind not working after defocus/minimization.

About the workaround:
For some reason, keybinds only works after virtual keyboard is opened at least once. I am currently fixing this by doing this:

SystemChannels.textInput.invokeMethod('TextInput.show');
SystemChannels.textInput.invokeMethod('TextInput.hide');

To be fair I am not too experienced in flutter but I tried various ways but none seemed to work. You might know better way to handle this. Let me know what to change to fix that if you have ideas. But this workaround has been perfectly working for me and keyboard doesn't even show up since show and hide happen one after another.

[codecov]

Codecov Report

Attention: Patch coverage is 44.92754% with 38 lines in your changes missing coverage. Please review.

Project coverage is 37.31%. Comparing base (2baa6e5) to head (f4317ed).
Report is 305 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/pages/editor/editor.dart	50.00%	24 Missing ⚠️
lib/components/toolbar/toolbar.dart	33.33%	14 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1132   +/-   ##
=======================================
  Coverage   37.31%   37.31%           
=======================================
  Files         108      108           
  Lines        8498     8524   +26     
=======================================
+ Hits         3171     3181   +10     
- Misses       5327     5343   +16     

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