Find and Replace Button and Toggle Option

[brandon1024]

Added a new button to toggle the find and replace pane, which is hidden by default. Also added an option to show or hide this button.

Fixes #313

[jakobi]

Thanx for the fix.

However there still seems to be a bug, kind of lacking one icon's width, with the pre-existing(?) now being "shared" between both clipboard and replace icons:

Prepare:

1. Open find+, hide both of these icons.
2. Close find+.

Reproduce:

1. Open find+, type LONGWORD
2. Click option icon.
3. Now repeat disable/enable one and or both icons.

Effect:

• X icon is being shifted to the right, start of LONGWORD is hidden.
• Sometimes you need to enable both icons, sometimes either one is sufficient.

Expected Effect: The size of the field should decrease, but the location of the 'X' should not be visually shifted, nor should the beginning of the regex become invisible.

Note: I already restarted firefox, albeit I'm testing this in my customized normal profile.