only activate required iced-features

[spamviech]

They are automatically activated for users of this crate.

My specific use-case:
For some reason, starting with iced-0.12 activating the wgpu-feature results in a "unwrap on a None-value" panic when executing on a rasperry pi.
Additionally, I don't want to activate the "tokio"-feature, since I don't use it for anything else (yet), and tokio isn't exactly small.

All examples are still compiling (added missing "icons"-feature where rustc was complaining).
However, I noticed some weirdness (not addressed here, to keep the scope small):

• wgpu requirement in examples:
  • panic on tab_bar (start) and multiple_modals (after button press)
  • sometimes starts with black window
  • color-picker bottom line stays empty
  • menu: when focusing an item, every other item is only a black box
• cuppertino_switch has weird behaviour (e.g. toggling both visually)
• split_scroller
  • panics when used with "wgpu" feature
  • horizontal scrolling is not shown in slidebar at the bottom
• folder for slidebar is named sliderbar (extra "r" in the middle)
• folder for widget_id is named WidgetIDReturn

[genusistimelord]

looks like a reasonable change, thank you