Start moving colors into themes

Using attrs instead of fixed colors it will be possible to style
everything and also dynamically grab the color where needed in code.
This is done with colorEmphasis as an initial test.

Also remove several unused colors.

OpenKeychain/src/main/res/values/attrs.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <declare-styleable name="CustomTheme">
+        <attr name="colorFab" format="color" />
+        <attr name="colorFabPressed" format="color" />
+        <attr name="colorEmphasis" format="color" />
+    </declare-styleable>
+</resources>