Create an event from shared or selected text (#306) (#396)

Shared text, or text picked with the new "Create event" entry in the selection menu, opens the new-event form prefilled. The share's subject becomes the title, or the first line if there is no subject, and the rest of the text goes into the description. Start and length use the usual defaults (next full hour, default duration). Calendula doesn't try to read a date out of the text.

Differences from the plan in the issue comment:
- The line used as the title is left out of the description, so it doesn't show up twice.
- Titles are capped at 255 characters. If a title gets cut, the full text stays in the description.
- Sharing a plain-text file no longer sends it to the .ics importer.
- A saved or dismissed prefilled form no longer reopens after rotating the phone. This also fixes it for "add to calendar" requests from other apps.
- If a second request comes in while a prefilled form is open, it replaces the form when nothing was changed. Otherwise the save / discard / keep editing prompt comes first.
- If Calendula was started just for the request, closing or saving the form takes you back to the app you came from.

Closes #306

Co-authored-by: Jean-Luc Makiola <business@jeanlucmakiola.de>
Reviewed-on: https://codeberg.org/jlmakiola/calendula/pulls/396

app/src/main/AndroidManifest.xml

@@ -151,6 +151,19 @@
                 <data android:mimeType="application/ics" />
             </intent-filter>
 
+            <!-- Text shared or selected in another app opens a prefilled new
+                 event (MainActivity.sharedTextExtrasOrNull, issue #306). -->
+            <intent-filter>
+                <action android:name="android.intent.action.SEND" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <data android:mimeType="text/plain" />
+            </intent-filter>
+            <intent-filter android:label="@string/process_text_create_event">
+                <action android:name="android.intent.action.PROCESS_TEXT" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <data android:mimeType="text/plain" />
+            </intent-filter>
+
             <!-- Let another app or widget (e.g. the Todo Agenda widget) launch us
                  to create a new event, the way the AOSP calendar accepts it:
                  ACTION_INSERT on the events *dir* mime type, carrying the new