feat(themes/powerline-icon): support OMB_THEME_POWERLINE_ICON_CLOCK for the date string in segment user_info

[elg]

powerline-icon has a hardcoded time format while apparently THEME_CLOCK_FORMAT is the way to go. If not set, it seems to still be %H%M%S or something like that so there is nothing broken here.

[RobLoach]

This approach is used elsewhere, so should be good.

[akinomyoga]

A subtle point is that THEME_CLOCK_FORMAT is already used to control the content of segment clock in the powerline themes. The clock segment can be enabled by specifying clock in POWERLINE_PROMPT. If we reuse the variable THEME_CLOCK_FORMAT to control also the format of the date embedded in the user_info segment, the content of clock segment is affected when the user wants to change the user_info, and the user_info is affected when the user wants to change the clock segment. If we would be going to provide the customizability, shouldn't we prepare a separate variable?

Also, the suggested change will affect the default behavior; %X %D and %H%M%S are different. The current default %X %D is not a random string but is used by the original author of this theme, so even if we provide customizability, we wouldn't want to change the default behavior unless there is a strong reason. Or we may ask the original author about the design change.

@TheWatcherMultiversal Do you have any opinion on the suggested change in the powerline-icon theme you contributed in #509?

[akinomyoga]

rebased and resolved a conflict.