Settings backup/restore + small fixes

Another stepping stone for settings sync, should enable easier debug for us and manual sync solution for the impatient. Also should make wiping localstorage less of a problem since it also backs up user highlights which don't have proper UI for them.

Major:

• Changed ImportExport component into a service since it's mostly logic/direct DOM-manip, not much of markup going on, makes it more reusable and avoid mess with slots
• Implemented settings save/restore function
  • New dropdown button in modal footer: save, save with theme, restore
  • Settings saved with array version - current version [1, 0] - works similarly to semver but in array, for future-proofing it will refuse to load settings with major version != 1 (i.e. [2, 0]) but will work different minor version, i.e. [1, 1].
  • Separate control for theme saving since theme data is quite heavy.
    • For future we might add dialog box asking if user wants to import theme as well if it's present
  • Restoring goes over supported properties and updates them using setOption with data found in file. Minor stuff:
• one more fix for "p's broken theme" fucking up theme editor and making it mostly unusable
• changed "Peek"/"Close" buttons to use icons instead, feels a bit more natural and less confusing given that there are now button in footer too.
• panel footers no longer use panel header colors, it just looked silly and didn't really work with some themes (redmond), cleaned up their styles a bit

src/modules/config.js

@@ -111,7 +111,16 @@ const config = {
   },
   actions: {
     loadSettings ({ dispatch }, data) {
-      Object.keys(this.state.config).forEach(
+      const knownKeys = new Set(Object.keys(this.state.config))
+      const presentKeys = new Set(Object.keys(data))
+      const intersection = new Set()
+      for (let elem of presentKeys) {
+        if (knownKeys.has(elem)) {
+          intersection.add(elem)
+        }
+      }
+
+      Object.keys(intersection).forEach(
         name => dispatch('setOption', { name, value: data[name] })
       )
     },