Imager Cache 'clear' buttons get weird text after cleaning one imagery source and then cleaning another one

[rickmastfan67]

Steps to reproduce:

1. Load more than 1 imagery source so that there are 2+ imagery cache sources.
2. Preferences > Imagery Preferences > Cache contents
3. Hit the 'clear' button on one of the 2+ sources.
4. Now hit the 'clear' button once again on another imagery source.

What happens:
A weird button with text shows up in place of the original 'clear' button on the sources you have 'cleared'.

What should happen:
The original 'clear' button should just be grayed out since there is '0' in the 'Object Count'.

I've attached a screenshot showing this 'weird text'.

Repository Root: http://josm.openstreetmap.de/svn
Build-Date: 2015-07-17 01:32:09
Last Changed Author: wiktorn
Revision: 8607
Repository UUID: 0c6e7542-c601-0410-84e7-c038aed88b3b
Relative URL: ^/trunk
URL: http://josm.openstreetmap.de/svn/trunk
Last Changed Date: 2015-07-16 21:55:32 +0200 (Thu, 16 Jul 2015)
Last Changed Rev: 8607

Identification: JOSM/1.5 (8607 en) Windows 7 64-Bit
Memory Usage: 359 MB / 1820 MB (216 MB allocated, but free)
Java version: 1.7.0_79, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM

Plugins:
- OpeningHoursEditor (31241)
- buildings_tools (31361)
- measurement (31289)
- mirrored_download (31294)
- osmarender (31241)
- reverter (31241)
- turnrestrictions (31241)
- undelete (31241)
- utilsplugin2 (31241)