Update Simplified Chinese localization

[udda]

Before next release, some strings need to be translated in Simplified Chinese. Here they are:

1. Multiload-ng (indicator)
2. Multiload-ng (system tray)
3. Direction of gradient
4. Under 'used' (like <i>htop</i> and some graphical task managers)`
5. Under 'cache' (like <i>free</i> and <i>top</i>)
6. Use IEC units (binary prefixes)
7. Count Slab cache:
8. Choose how to count Slab cache
9. If the window is larger than its minimum size, graphs will be scaled in proportion to the dimensions set.
10. Placement of system tray icons is controlled by the desktop environment. Graphs order may change.

@dongfengweixiao, could you?

[udda]

The list of untranslated strings is growing, Chinese localization seems abandoned. Chinese translators wanted!