Add Slot to Table Check All

[Minhyme]

Description

Provide a slot wrapping the existing b-checkbox for checking all rows.

<thead v-if="newColumns.length && showHeader">
    <tr>
        <th v-if="showDetailRowIcon" width="40px"/>
        <th
            :class="['checkbox-cell', { 'is-sticky': stickyCheckbox } ]"
            v-if="checkable && checkboxPosition === 'left'">
            <template v-if="headerCheckable">
                <slot name="check-all" :isAllChecked="isAllChecked" :isAllUncheckable="isAllUncheckable" :checkAll="checkAll">
                    <b-checkbox
                      autocomplete="off"
                      :value="isAllChecked"
                      :type="checkboxType"
                      :disabled="isAllUncheckable"
                      @change.native="checkAll"/>
                </slot>
            </template>
        </th>

Why Buefy need this feature

When using pagination, you can either use the default functionality of selecting all on the current page, or override this behavior by catching the check-all event. You cannot however do both. Providing a slot and binding the computed variables to that slot will allow people to create custom components which can achieve both goals.