location-entry: Replace fake eventbox buttons with real ones

Currently the reader mode and bookmark icons in the location entry
are made clickable with the mouse using event boxes. So they are
kinda "fake buttons" which aren't focusable at all using the
keyboard.

So to make it easier to access them with the keyboard, and to have
to way of using their features (keyboard focus and keyboard shortcut),
this commit replaces the event boxes with proper buttons, styled in
a way where the buttons are fully invisible to make it look like you
click or focus a clickable icon, not a button.
3 jobs for real-icon-buttons in 9 minutes and 14 seconds
Status Job ID Name Coverage
  Test
passed check-code-style #1344611

00:01:43

passed flatpak #1344612
flatpak

00:09:14

passed scanbuild #1344613
flatpak

00:06:02