Fix spacing for inline widgets

[DCsunset]

I have added a spacing setting for each widget that can control the spacing.
The Inline setting will remove the leading space, which is very useful for sub-widget in some blocks.
The Hidden setting will remove both the leading space and trailing space, which is useful for hidden blocks.

To demonstrate the usage, I have also fixed the spacing for nvidia_gpu block.

This PR should resolve the issue #800.

[ammgws]

Can you paste the before and after nvidia-gpu block output here (i.e. the raw output from i3status-rs when run from a commandline)?

[DCsunset]

Before:

{"background":"#000000FF","color":"#CCCCCCFF","full_text":"  GeForce MX150 ","markup":"pango","name":"f6a82f89f0f241dd838918b61a6deb6c","separator":false,"separator_block_width":0},
{"background":"#000000FF","color":"#CCCCCCFF","full_text":" 0 % ","separator":false,"separator_block_width":0},
{"background":"#000000FF","color":"#CCCCCCFF","full_text":" 52°C ","separator":false,"separator_block_width":0}

After:

{"background":"#000000FF","color":"#CCCCCCFF","full_text":"  GeForce MX150 ","markup":"pango","name":"547e37d81bae45cd9e96ecfc4713ac12","separator":false,"separator_block_width":0},
{"background":"#000000FF","color":"#CCCCCCFF","full_text":"0 % ","separator":false,"separator_block_width":0},
{"background":"#000000FF","color":"#CCCCCCFF","full_text":"41°C ","separator":false,"separator_block_width":0}

Leading spaces of both the temperature and utilization are removed.
Besides, when the name of the GPU is hidden, its trailing spaces will also be removed.

[DCsunset]

I have also fixed the comment and squashed it into the first commit.

[ammgws]

Apply to other blocks as well. Might as well fix them all at once.

[DCsunset]

I have fixed some blocks that I found with inconsistent spacing. Are there other blocks that need fixing as well?

[ammgws]

IIRC one would be the music block (the buttons at least).

[ammgws]

We can handle those in another PR

Thanks for your work!