UI styling revision

[martinfouilleul]

This simplifies the UI styling system and its API:

• Boxes don't inherit any styling from their parents. This avoids confusion about which attributes are inherited and which are not, and allows reasoning locally without caring as much about the context in which a widget will be called.
• All boxes are now blocks declared with oc_ui_box_begin()/oc_ui_box_end() (or the oc_ui_box()) macro
• Boxes don't take flags anymore. These have been moved to styling attributes or removed.
• Inside a box block you can directly set style attributes with oc_ui_style_set_xxx() functions, eg oc_ui_style_set_i32() for integer attributes or flags, oc_ui_style_set_color() for color attributes, etc.
• You can tag boxes with strings using oc_ui_tag().
• You can define a style rule with oc_ui_style_rule_begin()/oc_ui_style_rule_end() (or the oc_ui_style_rule()) macro. Rule patterns are now passed as strings and parsed by the UI system, without needing to setup the pattern structs. E.g. you can target box "A" with tags "x" and "y" with the pattern "A.x.y", or descendant B of A with "A B". Hover and active statuses are also tags, so for example you can target "A.hover".
• Inside a style rule you can use the same oc_ui_style_set_xxx() functions to set the rule's style.
• You can define style variables with oc_ui_var_set_xxx() functions (which create/overwrite variables) or oc_ui_var_default_xxx() (which only creates the variable if isn't already declared upstream).
• You can set a style attribute in a box or in a rule with a variable with oc_ui_style_set_var().
• Themes have been replaced with a number of default variables. Modifying the theme for a subtree amounts to changing the appropriate variables.

[martinfouilleul]

There's a couple other things I'd also like to cleanup in the rest of the UI:

• review and homogeneize the usage of hot/active/focus states
• Review the light theme
• Review mouse blocking
• Layout could also have another pass: some width attributes could use oc_ui_size instead of a float, some could be split into left/right/top/bottom components, etc
• Custom vars could be modifyable through rules.

But this can probably be left to a subsequent PR