Send caption with image and video

[bmarty]

Content

Add a composer in the media preview to be able to add a caption to image and video.

Motivation and context

Closes #2573

Screenshots / GIFs

sendCaption.mp4

Tests

• Sent an image or a video to a room
• observe that you can optionally write a caption, which will be then rendered in the timeline.

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes have been tested on an Android device or Android emulator with API 23
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly define what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR

[github-actions]

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

Attention: Patch coverage is 93.15068% with 10 lines in your changes missing coverage. Please review.

Project coverage is 83.02%. Comparing base (17ea2aa) to head (ddc4085).
Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
...oid/libraries/matrix/test/timeline/FakeTimeline.kt	0.00%	4 Missing ⚠️
...s/impl/messagecomposer/MessageComposerPresenter.kt	0.00%	0 Missing and 2 partials ⚠️
...ibraries/textcomposer/model/MessageComposerMode.kt	33.33%	2 Missing ⚠️
...mpl/attachments/preview/AttachmentsPreviewState.kt	75.00%	0 Missing and 1 partial ⚠️
...ent/android/libraries/textcomposer/TextComposer.kt	97.61%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3803      +/-   ##
===========================================
+ Coverage    82.90%   83.02%   +0.11%     
===========================================
  Files         1772     1772              
  Lines        44531    44614      +83     
  Branches      5225     5235      +10     
===========================================
+ Hits         36920    37040     +120     
+ Misses        5780     5745      -35     
+ Partials      1831     1829       -2     

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[everypizza1]

Just went through and tested with a screen reader: it works fine, although the text "optional caption" isn't read.

[ganfra]

Nice, thanks!