Add own background image upload

[w00000dy]

This adds the feature to upload your own background image.
If you select an image, it will be base64 encoded and then set as the background url.

Limitations:
The maximum size of the image depends on the maximum size of your browser's local storage. Usually browsers can store up to 5 MiB of local storage. If you try to use an image that is too large, you will get this error:

[blazoncek]

I'll share this image from @dosipod as I am not sure storing image in browser local storage is good or not, but I can't find the justification for increased file/code size.
I hope he doesn't object.

[w00000dy]

The code size increased by 176 Bytes.
Before:

RAM:   [==        ]  22.2% (used 72784 bytes from 327680 bytes)
Flash: [========  ]  82.4% (used 1295721 bytes from 1572864 bytes)

Now:

RAM:   [==        ]  22.2% (used 72784 bytes from 327680 bytes)
Flash: [========  ]  82.4% (used 1295897 bytes from 1572864 bytes)

In the picture, the image was stored on the ESP’s filesystem. Given the limited space available on the ESP’s filesystem, it can only accommodate very small images. Therefore, I believe that utilizing localStorage for storing the image would be a more efficient approach compared to the ESP’s filesystem.

As far as I’m aware, there are no significant drawbacks to storing an image in localStorage. The only potential issue is that the wledUiCfg JSON file might become quite large. However, if you need to inspect this JSON file, you can simply remove the background image beforehand. This should make the file more manageable.

[blazoncek]

In last 30 days we added almost 12kB into UI alone.
10.12.2023
const uint16_t PAGE_index_L = 32764;
10.1.2023
const uint16_t PAGE_index_L = 44491;

[w00000dy]

We moved simple UI to index page, this increased the size a lot.
On the other hand we have saved a lot by eliminating duplicate code in simple UI.
But I'll leave it up to you whether we merge this PR or not. 😁

[blazoncek]

True, and iro and color picker too.

[Aircoookie]

I really like this as it allows for custom backgrounds without utilizing very limited ESP resources or having to host and download the image from some external server!
Merging.

[w00000dy]

Thanks! Another point that came to my mind: Also, when using WLED in AP mode, you can't download the image from an external server because you don't have Internet access. So now you can also set your own background image in AP mode.