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.