Find the directory PREF correctly when installed in user directory

When installed with `pip3 install bcloud --user`, PREF should be $HOME/.local/share
use os.path.join to join paths
adjust width of the path button
Update screenshot
Merge pull request #79 from CzBiX/path-button-width

adjust width of the path button
remove build_translations
Add style classes to PathBox
Add i18n.sh
Merge pull request #78 from wzhd/master

安装在用户目录下时, 找到PREF目录的正确位置
Fixed: failed to load bcloud/bcloud.png; https://github.com/LiuLang/bcloud/pull/78
add history navigate button
add style for navigate buttons
refine code
show icon for root path
Merge pull request #80 from CzBiX/back-forward-button

Back forward button
Change icon size to SMALL_TOOLBAR
Show default app_info in popup menu
i18n updated
v3.5.6 released
Validate filepath