Show the difference between edited statuses (#3314)

* Show the difference between edited statuses

Diff each status against the previous version, comparing the different
HTML as XML to produce a structured diff.

Mark new content with `<ins>`, deleted content with `<del>`.

Convert these to styled spans in `ViewEditsAdapter`.

* Update diffx to 1.1.1

Fixes issue with diffs splitting on accented characters

* Style edited strings with Android spans

Don't use HTML spans and try and format them, create real Android spans.

Do this with a custom tag handler that can add custom spans that set the
text paint appropriately.

* Lint

* Move colors in to theme_colors.xml

* Draw a roundrect for the backoround, add start/end padding

Make the background slightlysofter by drawing it as a roundrect.

Make the spans easier to understand by padding the start/end of each one with
the width of a " " character. This is visual only, the underlying text is not
changed.

* Catch exceptions when parsing XML

* Move sorting in to Dispatchers.Default coroutine

* Scope the loader type

* Remove alpha

app/src/main/java/com/keylesspalace/tusky/util/StatusParsingHelper.kt

@@ -16,6 +16,7 @@
 @file:JvmName("StatusParsingHelper")
 package com.keylesspalace.tusky.util
 
+import android.text.Html.TagHandler
 import android.text.SpannableStringBuilder
 import android.text.Spanned
 import androidx.core.text.parseAsHtml
@@ -23,12 +24,13 @@ import androidx.core.text.parseAsHtml
 /**
  * parse a String containing html from the Mastodon api to Spanned
  */
-fun String.parseAsMastodonHtml(): Spanned {
+@JvmOverloads
+fun String.parseAsMastodonHtml(tagHandler: TagHandler? = null): Spanned {
     return this.replace("<br> ", "<br>&nbsp;")
         .replace("<br /> ", "<br />&nbsp;")
         .replace("<br/> ", "<br/>&nbsp;")
         .replace("  ", "&nbsp;&nbsp;")
-        .parseAsHtml()
+        .parseAsHtml(tagHandler = tagHandler)
         /* Html.fromHtml returns trailing whitespace if the html ends in a </p> tag, which
          * most status contents do, so it should be trimmed. */
         .trimTrailingWhitespace()